The proposed work involves study of animal models of hypersensitivity pneumonitis in the rabbit and guinea pig. Selective immunization, both active and passive, will be used to define immunologic reactions in the lung following the inhalation of aerosolized antigen. Light and electron microscopy will be utilized to study the histopathology in an attempt to define typical lesions and pathogenic mechanisms for different types of hypersensitivity reactions. Challenge with inhaled aerosolized antigen will be the main method of antigen challenge. Tracer studies of antigen fate in immunized and unimmunized animals will also employ radioactively labeled antigen solution introduced by bronchial catheterization.